You can flash a mod to your Nook that will swap the names of the internal and external storage -- so internal becomes ext_sdcard and external becomes sdcard.
I don't know how this works in practice as I have a 16gb Nook HD, so I didn't feel I needed to conserve space with the internal storage. So I didn't add that change to the one allowing sideloading when I modded the stock ROM recently.
But you can read about it here:
http://forum.xda-developers.com/show....php?t=2062613
See item #4 in the first post in that thread.
It might be worth a try for anyone with only an 8gb Nook HD.
--Pat